In 1991, a friction stir welding technique of joining metal materials such as aluminum alloys was established in the United Kingdom. This technique is for joining metal materials by pressing a cylindrical tool for friction stir welding having a small-diameter protrusion at a tip thereof against joint surfaces of the metal materials to be joined and rotating the tool for friction stir welding, generating frictional heat, and softening and plastically flowing the metal materials at a joint portion by the frictional heat (Japanese National Patent Publication No. 07-505090 (PTL 3)).
“Joint portion” herein refers to a joint interface portion where joining of metal materials by butting the metal materials or placing one metal material on top of the other metal material is desired. In friction stir welding, the metal materials near this joint interface are softened, plastic flow occurs, and the metal materials are stirred. As a result, the joint interface disappears and joining is performed. Furthermore, dynamic recrystallization occurs at the metal materials at the same time. Due to this dynamic recrystallization, the metal materials near the joint interface become fine particles and the metal materials can be joined with high strength.
When aluminum alloys are used as the above-mentioned metal materials, plastic flow occurs at a relatively low temperature of approximately 500° C. Therefore, even when the tool for friction stir welding made of inexpensive tool steel is used, little wear and tear occurs and frequent replacement of the tool is unnecessary. Therefore, in the friction stir welding technique, the cost required to join the aluminum alloys is low. Thus, in place of a resistance welding method for melting and joining aluminum alloys, the friction stir welding technique has already been in practical use in various applications as a technique of joining components of a railroad vehicle, a vehicle or an aircraft.
At present, the friction stir welding technique is mainly applied to nonferrous metals such as an aluminum alloy, a magnesium alloy and a steel alloy in which plastic flow occurs at a relatively low temperature. This friction stir welding technique is superior to the resistance welding method in terms of cost and time required for joining, strength of the joint portion, and the like. Therefore, there is a need for applying the friction stir welding technique to not only joining of the materials in which plastic flow occurs at a low temperature, but also joining of steel materials in which plastic flow occurs at a high temperature of 1000° C. or higher. In the following, various processing using the friction stir welding technique will be referred to as friction stir welding processing.
However, in friction stir welding under high temperature, the temperature of the stirred portion rises to a temperature near the melting point of the materials to be joined. Therefore, during joining, the materials to be joined react with a base material of the tool for friction stir welding, and thus, alloying of the base material progresses or the components of the base material dissolve in the materials to be joined. As a result, wear tends to progress. Furthermore, chipping or breaking tends to occur at a probe portion of the tool for friction stir welding, and thus, shortening of the tool life is a serious problem.
Japanese Patent Laying-Open No. 2003-326372 (PTL 1) discloses a technique in which a portion of a surface of a tool for friction stir welding that is in contact with materials to be joined is coated with a diamond film, thereby increasing the surface hardness. In Japanese Patent Laying-Open No. 2001-314983 (PTL 2), cemented carbide or the like having a hardness higher than that of materials to be joined is used as a material for a tool for friction stir welding.